The objectives of this project are to study the differentiation and maturation of human B lymphocytes. Ficoll-Hypaque separated lymphocytes and their purified B- and T- subpopulations were stimulated by Ionophore A23l87. It was found that: a) A23187 stimulates blast transformation of both T and B lymphocytes, b) Ionophore stimulates formation of lymphoblasts that produce IgM, c) Stimulation of IgM-positive blasts is better in the mixture of B and T subpopulations of lymphocytes than in B-cell enriched fraction, and d) Monocytes have an inhibitory effect on the appearance of IgM-positive cells but do not inhibit blast transformation. The inhibitory and/or enhancing influence of monocytes on the differentiation of B lymphocytes in other systems will be studied.